An optical splitter, which is a conventional optical device that utilizes MMI (multi-mode interference) asymmetric Y-branching, is described with reference to FIG. 21. The optical splitter includes an input-side optical waveguide 120, an MMI device 121, and an output-side optical waveguide 122. The output-side optical waveguide 122 is branched into a first output portion 123 and a second output portion 124 into which light is guided. The branching characteristics of the optical splitter can be controlled using the spacing g between the first output portion 123 and the second output portion 124 of the output-side optical waveguide 122 at the emission end of the MMI device 121 and the change Δ in the width of the incident end of the MMI device 121.
Single-mode light that is transmitted over the input-side optical waveguide 120 is split into zero-order mode (single mode) and second-order mode (multimode) light by the MMI device 121. Moreover, the length h of the MMI device 121 has been optimized to adjust the difference in propagation speed of the zero-order and second-order mode light so that the antinode portions in the center of the zero-order and second-order mode light cancel each other out. This leaves only the antinode portions of the second-order mode light at the emission end of the MMI device 121, with a spacing between them of several μm, and this light is split up to an effective splitting distance (>100 μm) by the output-side optical waveguide 122. Here, if the MMI device 121 is symmetrical, that is, if the change Δ equals zero, then the optical splitter splits the light equally at a splitting ratio of 1:1. If the MMI device 121 is asymmetric (change Δ is greater than zero), then the quantity of light that is branched to the side where the MMI device 121 is smaller, that is, the side of the second output portion 124, is decreased, causing an increase in the quantity of light that is propagated through the first output portion 123 and a corresponding decrease in the quantity of light that is propagated through the second output portion 124. Thus, the branching ratio can be controlled by controlling the amount of the change Δ. FIG. 22 shows the relationship between transmission loss and the branching ratio with respect to the change Δ. It should be noted that the branching ratio is the output of the first output portion 123 divided by the output of the second output portion 124. As is clear from FIG. 22, the change Δ is altered within a range of 0 to 5 μm, and thus the branching ratio is in a range of 1 to 3 and the transmission loss at this time is near 0.2 dB.
A conventional optical coupler is described next using FIG. 23. A conventional optical coupler that uses a Y-branched waveguide includes a first input-side optical fiber 131, a second input-side optical fiber 132, a coupling portion 137, in which a Y-shaped core 134 is formed on a substrate 133, and an output-side optical fiber 136.
If incident light of the same phase is incident on the first input-side optical fiber 131 and the second input-side optical fiber 132, then these two incident light beams are coupled to the coupling portion 137 from the first input-side optical fiber 131 and the second input-side optical fiber 132, and are coupled into zero-order mode light along the shape of the core 134 and emitted from the output-side optical fiber 136. The intensity of the light emitted at this time is the sum of the intensity of the two incident light beams incident from the first input-side optical fiber 131 and the second input-side optical fiber 132, and the optical coupler properly functions as a coupler.
As mentioned above, the branching ratio of a conventional optical splitter is determined by the shape of the MMI device 121. Thus after an optical splitter has been fabricated, that is, after it has been processed into a device, the branching ratio cannot be changed dynamically if the need arises.
Also, the branching ratio can be altered to a ratio of at most 3:1. Moreover, a centimeter-order length is required to expand the splitting distance of several μm at the output end of the MMI 121 up to a practical splitting distance at the output-side optical waveguide 122. This leads to an unavoidable increase in loss at the output-side optical waveguide 122 and an increase in size when producing the device.
On the other hand, like the optical splitter, it is not possible to dynamically change the coupling ratio of the optical coupler. Also, the coupling angle is at most about 2°, like the branching angle, and thus the optical coupler cannot be made shorter, leading to a large device.
If light is incident on only one of either the first input-side optical fiber 131 or the second input-side optical fiber 132 of a conventional optical coupler, then the zero-order mode of the incident light excites a zero-order mode and a first-order mode at a Y-shaped joint 137 and the first-order mode light is radiated from the output side. This causes the problem of emission from the output-side optical fiber 136 at only half the intensity of the zero-order mode of the incident light.
Moreover, conventional optical splitters and optical couplers employ optical waveguides, which means a high-degree of optical alignment and mode-shape matching is necessary between optical fiber and the optical waveguides, and expertise is required for assembly.